To operate properly, electronic circuitry (e.g., digital logic) must be fault free. Accordingly, numerous techniques have been developed to identify circuit faults such as level sensitive scan design (LSSD) testing, quiescent current (Iddq) measurements and delay fault measurements.
During LSSD testing of a circuit, a logical voltage pattern is applied to the circuit and the resulting logical circuit outputs are observed. The resulting logical circuit outputs then are compared to the logical expected values for the circuit, and a fault is identified by a discrepancy therebetween. LSSD testing thus employs a simple binary comparison that provides no information about the internal voltage potentials of a circuit. Faults which degrade a circuit's internal voltage potentials without affecting the circuit's logical outputs (i.e., potential faults), therefore, are unidentifiable by binary testing schemes such as LSSD testing.
During an Iddq measurement, a DC voltage pattern is applied to a circuit, the power supply current supplied to the circuit is measured and the resulting power supply current is compared to an expected power supply current in order to identify faults. Similarly, during a delay fault measurement, the voltage pattern applied to a circuit's inputs is changed from one voltage pattern to another, the time required for the circuit's outputs to change states in response thereto (i.e., the circuit delay) is measured and the resulting circuit delay is compared to an expected circuit delay in order to identify faults. While both Iddq and delay fault measurements are analog in nature (e.g., measuring an analog power supply current and an analog circuit delay), neither measurement provides information about a circuit's internal voltage potentials. Additionally, as electronic circuits progress into the deep sub-micron regime, larger sub-threshold leakage currents result diminishing the usefulness of Iddq measurements. Accordingly, a need exists for an improved method and apparatus for identifying circuit faults.